This invention relates to a method for reducing the rate of deterioration of perishable horticultural produce and in particular relates to the use of nitric oxide (NO) in such a method. More particularly, the horticultural produce to which the method is applicable, is fruit, vegetables and/or flowers. The method is applicable during post-harvest handling, storage and marketing.
Fresh horticultural produce such as fruit, vegetables and flowers, are highly perishable and degrade rapidly after harvest. This results in substantial quantities of harvested produce being discarded at some point in the postharvest chain or being marketed at a discounted price. The overall outcome is a reduction in the quality of the available food.
There are many causes of deterioration in horticultural produce but an important factor is the accumulation of ethylene in the atmosphere around such produce. It is well known that ethylene accelerates deterioration in horticultural produce. This may be through initiation of premature ripening, acceleration of the loss of green colour, and/or the development of yellowing. It may also result in the increase in microbial growth, induction of physiological disorders and/or the development of undesirable flavours and texture. It may also cause the promotion of leaf petal damage in the case of flowers. Thus, the net effect of ethylene is the enhancement of senescence and rotting of non-climacteric produce and the initiation of ripening of climacteric produce.
Ethylene is generated by all horticultural produce. Thus, it can accumulate to relatively high concentrations in the confined spaces of postharvest containers or storage chambers. Ethylene can also be derived from a wide range of other sources related to the incomplete combustion of fuels as in motor cars, fork lift vehicles or equipment powered by an internal combustion engine. Ethylene can also be derived from the cross contamination with ethylene produced from one commodity accumulating around other commodities held in the same storage chamber.
Traditionally, a threshold concentration of 0.1 xcexcL/L ethylene was considered to be the safe limit of ethylene exposure. However, recent studies in Australia on a wide range of horticultural produce has shown there is no safe limit of ethylene exposure and that concentrations above 0.005 xcexcL/L produce a deleterious response. These findings have emphasised the benefit that can be obtained from either preventing the accumulation of ethylene around produce or from inhibiting the action of ethylene that does accumulate around produce.
Nitric oxide has been shown to be metabolised by growing vegetative plants. The application of nitric oxide at low concentrations has also been found to reduce the production of ethylene by young, growing vegetative cells from leaf epidermis and foliar cells and assist in growing plants cope with water stress. Conversely the application of nitric oxide at higher concentrations enhances ethylene production and reduces the ability of growing plants to cope with stress. Prior to now, nitric oxide has not been considered for use on mature plant organs such as fruit, vegetables and flowers.
It has been surprisingly found that nitric oxide can be used to extend the postharvest life of perishable horticultural produce such as fruit, vegetables and/or flowers, by reducing the rate of deterioration of such produce. This is achieved by nitric oxide through a reduction in the rate of rot development, loss of green colour and associated enhancement of yellowing, ripening of fruits, and expression of chilling injury.
It has further been surprisingly found that nitric oxide can inhibit transpiration, that is, evaporation of water from produce. Water comprises 85-95% of the composition of virtually all horticultural produce and after harvest water is continually evaporated into the atmosphere due to humidity differentials. It is important that the loss of water is minimised as it plays a crucial role in the maintenance of appearance and texture in the produce. The loss of about 3% water induces visible changes in produce quality such as wilting and shrivelling. The price paid for wilted and shrivelled produce is thus severely discounted.
The problem of loss of water is important for all produce but is particularly serious for produce such as leafy vegetables and flowers which have a high surface area:volume ratio. Our research has found that the nitric oxide treatment inhibits the rate at which water is evaporated from produce during subsequent storage in air.
Thus, according to this invention there is provided a method for reducing the rate of deterioration of perishable horticultural produce by fumigating said produce, postharvest, with nitric oxide.
Typically, the perishable horticultural produce is fruit, vegetables and/or flowers. Ideally, the nitric oxide should be applied within a few hours after harvest before any undesirable postharvest changes have occurred.
Suitably, the container housing the produce to be treated by the method of this invention is purged with an inert gas such as nitrogen, prior to introduction of the mixture of nitric oxide and inert gas, as nitric oxide is oxidised in the presence of oxygen.
Fumigation of horticultural produce with nitric oxide is only required for a relatively short period. Periods of 1-24 hours with nitric oxide concentrations of 0.1-200 xcexcL/L have been found to be effective. The application of 5 xcexcL/L nitric oxide for 2-6 hours is an effective treatment combination for many types of produce while for some produce a 12-24 hr exposure is acceptable. The optimal fumigation treatment, that is, the nitric oxide gas concentration and the time of exposure, can be varied by raising the nitric oxide concentration and reducing the exposure time, or vice versa.
Variation is also dictated by the produce type and length of the subsequent marketing period. The upper time limit for fumigation is determined by the sensitivity of individual produce to be held in an oxygen free atmosphere before excessive anaerobic metabolism leads to undesirable effects.
Nitric oxide fumigation should occur in an atmosphere that is completely free of oxygen as small amounts of oxygen will react with nitric oxide and thereby render it inactive. Typically, nitric oxide is used in conjunction with an inert gas. Nitrogen is the only inert gas that is relevant commercially due to its cost. Other gases such as carbon dioxide and argon are technically acceptable.
However, it has been found that the presence of up to 2% oxygen in the fumigation chamber atmosphere has little degradative effect on nitric oxide if the fumigation time is 2 hours or less. It is also possible to increase the initial concentration of nitric oxide to allow some loss of nitric oxide during fumigation. Concentrations of nitric oxide up to 200 xcexcL/L have been successfully applied.
After fumigation, the produce can be held in a normal air atmosphere. The beneficial effects of the fumigation treatment are retained after the nitric oxide is removed. Horticultural produce can be fumigated at any time after harvest but maximum effectiveness is obtained if it is applied soon after harvest.
The fumigation treatment thus provides an additional advantage of treating fresh horticultural produce soon after harvest and then allowing it to move into the normal postharvest chain without the need for any special storage conditions.
The nitric oxide for use in this invention may be purchased commercially in a cylinder of a mixture of nitric oxide and inert gas, or generated in a sealed chamber containing the produce in an inert atmosphere. There are many methods by which nitric oxide can be generated chemically. One such example of a chemical reaction between an acidified solution of potassium nitrite and potassium iodide. In this instance, two solutions are mixed: Solution A is 0.1M potassium iodide +0.1M sulfuric acid; and Solution B is 50 xcexcM potassium nitrite.
The following describes one embodiment of the invention which should not be considered limiting on the scope thereof.
Horticultural produce was placed in a vessel, sealed and purged with nitrogen free of any oxygen. A mixture of nitric oxide (in the range of 0.1-200 xcexcL/L) in oxygen-free nitrogen was then introduced and the produce fumigated with this mixture. There are no upper or lower limits to the ratio of produce weight to total volume of the fumigation chamber. The critical issue is that the desired concentration of nitric oxide is maintained throughout the fumigation period. A fumigation time of 2 hours with 5 xcexcL/L nitric oxide was found to be effective for many types of produce but the optimal fumigating conditions for individual types of produce need to be experimentally determined.